Vasopressin (VP) is essential for regulation of water balance. More recently other important functions have been proposed, e.g. a possible role in the genesis and/or maintenance of hypertension and various behavioral effects. Regulation of VP release is complex and some aspects are difficult to study in vivo. Multiple substances affect VP release and it has been difficult to determine where these agents have their effect, i.e., in hypothalamus (HT), directly on neurohypophysis (PP), or both. To address this question, a new in vitro model, the compartmentalized rat hypothalamo-neurohypophysial system (CHNS) has been developed. In this preparation, HT and PP are maintained in organ culture in separate compartments. The intact infundibular stalk passes through a fluid-tight barrier which separates the 2 sides. This represents a major advance over isolated PP lobes. VP release is measured by radioimmunoassay (RIA). This system has stable VP release rates for 3-4 days in vitro, and responds appropriately to osmotic stimulation and acetylcholine administration on HT side. Several substances known to affect VP release are endogenous to HT, PP, and/or pars intermedia (PI). These include Acetylcholine, Angibtensin II, Dopamine, Gamma-Aminobutyric Acid, Histamine, and many other substances which also affect neuronal activity. Because of their anatomical distribution it is likely that some of these may affect VP release either in HT, PP, or both. Specific goals are: 1) to continue to characterize the function of CHNS under basal and stimulated conditions, 2) to determine the effects of each of the aforementioned agents separately and in various combinations, and to localize their general site(s) of action to HT and/or PP, 3) to evaluate whether VP affects its own release, 4) to characterize the morphology of CHNS and to assess the status of axonal transport mechanisms, and 5) to investigate whether VP synthesis is still occuring in vitro.